This invention relates to an abutment with seismic restraints, and more particularly, to an abutment especially adapted for use with a bridge superstructure, the abutment including integral lateral containment elements which prevent undesirable differential lateral shifting or movement of the bridge superstructure during a seismic event.
Engineers throughout history have developed bridge designs which have resulted in literally thousands of different types of bridge constructions. Prime considerations in bridge building are to span a gap in the most safe, efficient, and cost effective manner. While many bridges may be aesthetically pleasing and functional considerations have not been the driving factor for their design, a great majority of bridges are designed primarily for their functional purpose.
In all industrial nations, there are specific engineering standards which must be met in the design and construction of a bridge. Bridges are intended to be structures which will not collapse during normal use, as well as foreseeable natural acts such as storms or other natural phenomena. Thus, bridges are designed to account for not only loading conditions which are always present (e.g., the dead load of the bridge and the live loads transmitted by users of the bridge), but also loading conditions created by wind, snow, or other natural weather conditions. One particularly devastating type of natural event which continues to cause destruction of even the most well designed bridges are earthquakes. While a bridge designer in some geographical locations may be forced to comply with certain standards to handle an earthquake, recent history has shown that a great majority of bridges are not designed to adequately withstand an earthquake even when the bridge design satisfies local engineering standards. As well understood by those skilled in the art, earthquake damage is primarily due to lateral shifting of manmade structures. Particularly in bridge designs, there is little consideration given to designing bridge abutments in order to minimize the damage which can be created by an earthquake.
Inherent in any bridge design is the desire to limit the lateral or transverse movement of the bridge superstructure so that the bridge superstructure moves as a single unit as opposed to a number of separate parts. Accordingly, there are numerous types of lateral supports (e.g., gussetts or baffles) found within bridge superstructures which extend substantially perpendicular to the girders of the bridge superstructure. The girders typically run in the direction of the gap to be spanned. During a seismic event like an earthquake, a great majority of the lateral force of the bridge superstructure is directly transferred to the bridge abutments. While the bridge girders, overlying decking and roadway may be able to withstand a particular seismic event, weakening or destruction of the bridge abutments will result in bridge superstructure damage or destruction simply due to the fact that the bridge superstructure is no longer properly supported at its respective ends by the abutments. Whether a bridge superstructure includes a single span or has multiple intermediate supports between the bridge abutments, preventing damage to the bridge abutments is critical in ensuring that the bridge superstructure can adequately withstand a seismic event.
In accordance with the present invention, an abutment is provided for use with a bridge superstructure wherein the abutment includes lateral containment elements which reinforce the abutment to prevent undesirable differential lateral displacement or movement of the bridge superstructure during a seismic event. The term xe2x80x9cbridge superstructurexe2x80x9d as used herein refers to the major structure of the bridge which rests upon the abutments and rests upon any intermediate supports. As understood by those skilled in the art, the bridge superstructure includes the girders, lateral supports, decking, and the roadway above the decking. It should also be understood that subsequent reference to the term xe2x80x9cbridgexe2x80x9d herein more specifically refers to the bridge superstructure. The differential lateral displacement or movement of the bridge during a seismic event refers to the additional lateral shifting or movement which is experienced by the bridge superstructure during a seismic event due to the fact that the bridge is not adequately restrained in its connection to the abutments. That is, during a seismic event the abutments themselves will also laterally shift in response to the shifting movement of the earth during the seismic event, and the differential displacement or movement of the bridge superstructure constitutes not only the additional magnitude of displacement of the bridge superstructure, but can also refer to the out of phase oscillation of the bridge in comparison to the abutments.
The lateral containment elements can be constructed of varying materials and can be represented herein as differing embodiments of the current invention. In a first embodiment of the invention, the bridge abutment may include lateral containment elements made of mechanically stabilized earth which extends laterally away from each lateral side or end of the sill of the abutment. The mechanically stabilized earth is confined within an area between the lateral ends of the sill and wing walls or wing extensions which extend away from each end of the facing wall of the abutment.
In a second embodiment of the invention, the lateral containment elements are reinforced concrete blocks which may be pre-fabricated for the particular bridge design, or may be poured in place at the job site. The concrete blocks may be further reinforced by the use of one or more micropiles which have an upper end encased within the concrete block and a lower end which extends below the abutment into the ground.
In yet another embodiment of the invention, the lateral containment elements are a plurality of steel piles or beams which are driven into the ground or emplaced in pre-drilled holes which abut or are placed directly adjacent to each lateral end of the sill. These steel piles are sized and spaced from one another in a manner which provides the desired level of lateral restraint to the superstructure of the bridge.
With respect to use of concrete blocks as the lateral containment elements, the concrete blocks may be placed on a flat surface of the abutment directly adjacent the sill, this flat surface preferably being at the same height as the sill. Alternately, the concrete blocks may extend below the level of the sill and into the ground or the mechanically stabilized earth beneath the flat surface. For concrete blocks which include a portion which extends below the flat surface, the portion extending below can be considered a shear key which further stabilizes the concrete block. Additionally, one or more micropiles could also be contained within the shear key and having a lower end which extends further below the shear key to provide yet additional anchor stabilization to the concrete block.
An additional feature of the invention, which may be incorporated for a bridge spanning a river which is subject to erosion by scour, is the use of a plurality of micropiles which are placed externally of the facing wall of the abutment and which extend downwardly into the ground below the river bed. In short, these scour micropiles help to stabilize the earth around the abutment and to prevent scour which could result in an undercut of the river channel with respect to the facing wall of the abutment.
Yet another feature of the invention which may be incorporated within the various embodiments is a modified bearing member of the sill which can extend into each of the lateral containment elements, thus providing further strength to the abutment design and enhancing the ability for horizontally transmitted loads from the bridge superstructure to be absorbed within the abutment.
For each of the embodiments of the invention, lateral stability and strength is provided to the abutment by lateral containment elements that are of simple yet effective design. Traditional bridge abutment designs may be supplemented by incorporating the lateral containment elements without having to substantially redesign the entire bridge abutment. A minimum amount of material and labor is required to install the lateral containment elements thus enhancing the ability of the invention to modify traditional bridge abutment designs.
Other features and advantages of the invention will become apparent from a review of the following description, taken in conjunction with the accompanying drawings.